The present invention relates to an apparatus and method for recording acoustical holograms utilizing at least one ultrasonic transmitter for irradiating an object to be recorded, one sound receiver for receiving a reflected sound, and an apparatus for converting the received sound signal and transforming it into an optical signal for recording as a hologram.
In acoustical holography, an object wave field emanating from an object is scanned by a receiving transducer which converts the received signal into electrical output signals which are in turn converted into light signals which are recorded row-by-row as holograms on a light sensitive medium. In order to spatially separate the image of the object from the non-diffracted light and the conjugate image during reconstruction of the hologram, optical holography during recording of the holograms normally uses a reference wave which is directed at an angle to the object wave.
In acoustical holography, the reference wave is usually added electronically to the electrical signal of the object wave since linear receivers are employed to record the sound waves.
A fundamental problem in recording acoustical holograms consists in finding a suitable sound transducer for the reception of the ultrasonic waves. The transducer must be able to scan the sound field with a high degree of sensitivity over as large as possible area. Hitherto, a particularly high degree of sensitivity could be achieved only by employing piezoelectric sound transducer. When using piezoelectric sound transducers, the sound field is electronically scanned either with a matrix of sound transducers or else is received or scanned spot-by-spot or row-by-row by mechanical translation of the sound transducer over the field.
However, due to the large number of sound transducers which are required, the use of a matrix of sound transducers is expensive. The mechanical scanning of the sound field by translating a sound transducer is a time consuming operation and rules out the possibility of making a holographic recording of a moving object.